Poly(vinyl alcohol) (PVOH) is a water soluble polymer prepared by acid- or base-catalyzed alcoholysis. It is desirable to have access to PVOH films of variable solubility for applications that have different dissolution temperature requirements. For example pouches containing detergent for consumer cleaning applications are preferably soluble in cold water, at 10° C. or above. At the opposite extreme, PVOH laundry bags preferably only dissolve in hot water.
The solubility of PVOH can be controlled by the degree of hydrolysis. For water-soluble films the hydrolysis is typically 80-100%. A common grade used for cold-water soluble film is 88% hydrolyzed PVOH. As the hydrolysis increases from 88 to nominally 100%, the water temperature required for dissolution increases.
To influence solubility of PVOH, functional comonomers can be incorporated in the PVAc polymerization to modify solubility by their disruption of PVOH crystallinity and/or their hydrophobic or hydrophilic nature. See, e.g., Polyvinyl Alcohol-Developments; Finch, C. A., ed., John Wiley & Sons: New York, 1992, pp 361-402 (copolymerizing an acrylate or methacrylate ester with vinyl acetate, which, after transesterification, results in a lactone functional group on the polymer chain).
In the extrusion of PVOH, grades of PVOH having the degree of hydrolysis that confer the desired solubility of the film are utilized. In some instances PVOH resins with the desired hydrolysis and molecular weight are not commercially available. One can employ blending strategies in this instance, but in many instances this may necessitate a compromise in other properties and the final dissolution temperature may still be dominated by the component with the higher dissolving temperature. Therefore it would be desirable to have alternative methods to fine-tune the solubility of resins during processing.
PVOH may be made into film by many known methods such as solution-casting or melt extrusion techniques to produce water-soluble films. Such films are also useful as packaging materials in a number of uses, for instance, where a package of chemical is premeasured and sealed, but which subsequently liberates its contents when placed in water. Examples include laundry detergents and sanitizing chemicals for hot tubs, etc.
PVOH film is also an oxygen barrier and can be used in multilayer films such as a PVOH film sandwiched between hydrophobic films. Such a composite film is resistant to water but retains oxygen barrier properties and can be useful for packaging materials that require protection from oxidation, for instance in the packaging of certain foods and medical products.
The melt extrusion process for making PVOH films is carefully managed, as PVOH exhibits a tendency to undergo decomposition in an extruder, due to screw shear and residence time under heat. The decomposition, believed to begin at less than or about 200° C., is evidenced by undesirable properties such as non-uniform properties, discontinuities in the film, coloration, embrittlement, cross-linking, loss of biodegradability, and loss of water solubility.
Plasticizers are used with PVOH in melt extruding to lower extrusion temperatures and overcome the inherent thermal instability or in softening the PVOH to provide softer, more flexible articles. Polyols such as glycerol are commonly utilized for this purpose. Glycerol triacetate has been utilized as a plasticizer with 88% hydrolyzed PVOH in WO98/39382, but glycerol triacetate is a poor plasticizer for PVOH of higher hydrolysis, such as >95% or >98%. It is known that as the hydrolysis level of PVOH increases, it becomes less compatible with non-polar molecules such as triacetin.
It is desirable to prepare biodegradable PVOH plasticized with biodegradable plasticizers that specifically assist in lowering the polymer melt temperature and the extrusion temperature and at the same time maximize water solubility by minimizing crystallinity. It is also desirable to extrude fully hydrolyzed PVOH, since some applications require reduced water solubility.